The present invention generally relates to methods and devices for power generation in airborne vehicles, and more specifically for airborne vehicles operating at high altitudes and/or low speeds, where the ambient air pressure is low and the work required to compress ambient air for a gas turbine or reciprocating engine is excessive.
Conventional airborne vehicles achieve flight by using a fuel, such as jet fuel, combusted with air in a gas turbine engine to generate sufficient thrust to enable the wings to develop lift in order to keep the vehicle aloft. The gas turbine engine may also be used for generating power for onboard needs, such as, for example, hydraulic power for control surface actuation and electrical power for avionics equipment. Such gas turbine engines used to generate power for thrust, lift, and onboard power requirements typically operate in an open loop Brayton cycle. It has been found that this approach provides an acceptable means for powering an airborne vehicle in terms of fuel consumption, weight, and cost, and this approach is thus a standard method of powering air vehicles.
In a conventional, recuperated gas turbine engine 100 operated in an open-cycle, as illustrated in FIG. 1, the compressor 110, turbine 120, and generator 130 are coaxially mounted on a common central shaft 140, or the generator is driven by the common shaft turbine-compressor via a gearbox. The compressor 110 compresses air for the combustor 160 after it has been heated in the recuperator 180. The heated compressed air is mixed with fuel in the combustor 160 where it is then ignited and burned. The combustion products are then expanded in the turbine 120 which drives the compressor 110 and generator 130. This cycle is opened between the recuperator outlet 150 from the low-pressure side of the recuperator 180 and the compressor inlet 170. Thus ambient air that may have been filtered, cooled, or otherwise treated enters the compressor 110, and the products of combustion are discharged out the recuperator outlet 150 on the low-pressure side of the recuperator 180.
However, a new class of airborne vehicles is currently being studied for surveillance and communications relay applications. Such applications require long flight durations, in terms of days and months instead of hours, at high altitudes. Airborne vehicles of this class are generally slow-moving, lighter-than-air vehicles, which require little or no thrust for lift and minimal thrust for station keeping. Weight must be kept at a minimum in order for these vehicles to stay aloft for these extended periods of time. Because the vehicles are slow-moving, they do not require large engines for thrust and consequently feature much smaller engines with correspondingly low onboard fuel storage capacities. These features also serve to minimize the weight of such airborne vehicles. However, engines having low or no thrust requirement generally are not able to provide electrical power to operate onboard avionics equipment at high efficiency or low specific fuel consumption.
Selection of a system for generating onboard electrical power for lighter-than-air airborne vehicles can be difficult. Conventional open-loop Brayton cycle systems do not offer particularly high thermal efficiency when the available thrust provided by the engine exhaust is discounted. Furthermore, at the low ambient pressures found at altitudes above 50,000 feet, both the power and the efficiency of these conventional open-loop Brayton cycle systems are greatly diminished. Even at high speeds in excess of 550 knots with effective (90%) ram recovery efficiency, a typical gas turbine engine provides at 50,000 feet only 14% of the thrust available at sea level, as discussed of the reference work entitled “Aircraft Turbine Engine Technology” by Irwin Treager, page 101, FIG. 3-16, which is incorporated herein in its entirety by reference. At lower airspeeds, or without ram recovery, there is essentially no output shaft power available from gas turbine engines operating in an open-loop Brayton cycle at altitudes exceeding 50,000 feet.
The problems attendant with high altitude operation of Brayton cycle gas turbine engines have been addressed in the prior art. For example, U.S. Pat. No. 4,759,178 describes an auxiliary power unit comprising a gas expansion motor and a Brayton cycle gas turbine engine which jointly power a common load. The gas expansion motor is used initially to start the gas turbine, which is powered by standard JP-type fuel. This system employs two separate motors and therefore would present a weight penalty to a light, high altitude airborne vehicle.
U.S. Pat. No. 4,067,189 employs another such engine combination, where a closed loop Rankine cycle and an open-loop Brayton cycle are used in conjunction with one another. Again, because two engines are used, there would necessarily be a weight penalty for light, high altitude airborne vehicles.
U.S. Pat. No. 5,012,646 relates to an engine having a means for pre-cooling the air between the compressor and the combustor of the turbine. This system requires a significant expenditure of power to compress ambient air, especially at high altitudes.
U.S. Pat. No. 5,309,029 provides a turbine engine having a clutch enabling the compressor section to be decoupled from the turbine section, so that the engine can be more efficiently operated using stored oxidizer at a higher altitude but coupled so that the compressor can enable the engine to provide greater power at lower altitude. Again, the presence of a clutch and an additional oxidizer source creates a weight penalty for light, high-altitude airborne vehicles.
An Otto cycle piston engine is a more attractive alternative than an open Brayton cycle engine at low power levels and low airspeeds. The overall thermal efficiency of an Otto cycle piston engine is 20-30%. However, the power output of a piston engine is approximately proportional to the engine intake pressure. At high altitudes above 50,000 feet, even a modestly powerful piston engine would be quite large and heavy. The engine size can be reduced with a turbocharger or a supercharger, but these devices adversely affect the overall engine efficiency. Furthermore, the storage weight of the onboard fuel required for long duration flights would exceed the practical weight requirements for a lighter-than-air airborne vehicle.
Another alternative would be the use of nuclear power with a gas turbine engine operating with a closed loop Brayton or Rankine cycle. This combination would provide substantially longer operating duration, but is considered unattractive from an environmental standpoint.
Solar power may also be used for electrical power generation for lighter-than-air airborne vehicles. Such solar systems might involve the use of photovoltaic cells or a solar-heated Brayton or Rankine cycle system, coupled with batteries for power at night. Although an onboard fuel supply is not required by such a system, the solar collector required to provide electrical power even at modest power levels would be quite large, and the batteries would be unacceptably heavy.
As can be seen, there is a need for a lightweight system for generating power for a lighter-than-air airborne vehicle. The generating system should operate without compression of ambient air that normally results in high specific fuel consumption, and without the use of an on-board oxidizer, the weight of either of which would prevent the airborne vehicle from staying aloft for several days or even months.